1. Field of the Invention
The present invention relates to a photographic lens structure having its focusing lens moved by drive means such as a motor and, more particularly, to a focusing lens movement compensating device for use in an interchangeable lens, which is coupled to a focal point detector for detecting the displacement of the position of a focused image from an intended focal plane, which device properly transforms such movement of the focusing lens to correspond to said displacement.
2. Description of the Prior Art
In the prior art, there are known many photographic lenses in which a focusing lens forming part (or all) of a photographic lens is adapted to be moved by the rotating force of a motor. In these photographic lenses, there is also known an autofocusing device in which said photographic lens is interchangeable and in which the focal point detector is attached to the camera body to detect the amount of out-of focus from the intended focal plane or an equivalent position thereby to know the displacement between the intended focal plane and the position of the focused image so that the focusing lens is automatically moved by the focusing operation in accordance with that displacement by a motor mounted in the camera body or the interchangeable lens. In that auto-focusing device, when the aforementioned displacement has a certain value, this value is transformed through a coefficient into the number of rotations of the aforementioned motor movement of the focusing lens. Here, the movement required of the aforementioned focusing lens for proper focus for a certain displacement is different depending upon the individual interchangeable lens. In other words, when a certain displacement is obtained by the focal point detector, the focus can not always be obtained if the focusing lens is moved a constant distance for all interchangeable lenses, but the focusing lens has to be moved by distances intrinsic to the respective interchangeable lens selected. Hence, the coefficient for transforming the displacement into the movement of the focusing lens is intrinsic to each interchangeable lens, and the intrinsic coefficient has to be given as information to the auto-focusing device each time the interchangeable lens is replaced by another.
A prior art device constructed to establish the intrinsic information for each interchangeable lens is disclosed in Japanese Patent Laid-Open No. 57-165821. In this publication, there is disclosed an information establishing device in which a pattern is turned by a gear rotated when the focusing lens is moved so that pulses generated by the light incident on the pattern and reflected therefrom. Since the pattern is formed alternately with highly reflective portions and lowly reflective portions, the reflected light is discontinuous and is received by a photo-receiver so that it may be converted into an electric current. At this time, by changing the pitches of the highly and lowly reflective portions of the pattern or by selecting the angular velocity of the pattern, the number of pulses generated in a manner to correspond to the certain movement of the image can be made identical, when the focusing lens is to be moved for a certain displacement until it is focused, even if the movements are different for the respective interchangeable lenses.
According to this construction, the device for generating signals such as the pulses has to be attached to each interchangeable lens with the disadvantage that the interchangeable lenses have complicated structures but also to make it impossible to solve the following problems.
Specifically, the interchangeable lens to be used in the camera is subjected to various types of focal adjustments, if necessary. However, the aforementioned coefficient is not always constant in dependence upon the types of the focal adjustment. FIG. 4 shows the individual types of the focal adjustment in the interchangeable lens. FIG. 4(a) shows total lot-off type in which the all the components of the lens structure are integrally axially shifted or let off; FIG. 4(b) shows the internal focusing type in which only the central group is axially shifted or let off while the front and rear groups remain stationary; and FIG. 4(c) shows the rear focusing type in which the front group is immovable but the rear group is axially shifted or let off. In these Figures, letter o indicates lens optical axis, and letter F indicates a focusing lens group having a focal lens f.sub.F. In FIGS. 4(b) and (c), letter A indicates the front group having a focal distance f.sub.A. In FIG. 4(b), letter B indicates the rear group having a focal length f.sub.B. Here, the infinitesimal movement of the focusing lens group F is indicated as .DELTA.d whereas the infinitesimal displacement of the image is indicates as .DELTA.L, and the equation of the relation between .DELTA.d and .DELTA.L is to be determined.
First of all, for the total let-off type, as is understood from FIG. 4(a), the relation between the infinitesimal movement .DELTA.d of the lens group and the infinitesimal displacement .DELTA.L of the image substantially satisfies the following equation:
(.DELTA.d/.DELTA.L)=1 PA1 .beta..sub.FX.sup.2 indicates the magnification of the focusing lens group at X.
Next, turning to FIG. 5, the equation expressing the relation between the infinitesimal movement .DELTA.d of the focusing lens group F and the instant displacement .DELTA.L of the focal point in the general state of the internal focusing type interchangeable lens is to be determined. Then: the distance between the object point and the principal point of the focusing lens group F having a negative power is indicated at S.sub.F ; the distance between the image point and the principal point as S.sub.F '; the distance between the focal point and the object point as n; the distance between the focal point and the image point as n'; and the magnification as .beta..sub.F. From the relations of S.sub.F =f.sub.F +n and S.sub.F '=f.sub.F -n' and from the well-known formula of .beta..sub.F =-f.sub.F /n=n'/f.sub.F, the following equations are obtained: EQU S.sub.F =.function..sub.F (1-(1/.beta..sub.F) .circle.1 EQU S.sub.F '=.function..sub.F (1-.beta..sub.F) .circle.2
If D.sub.F =S.sub.F +S.sub.F ', .DELTA.D.sub.F =.DELTA.S.sub.F +.DELTA.S.sub.F +.DELTA.S.sub.F ', Into this equation, the equations 1 and 2 are substituted after they have been differentiated: ##EQU1## Since the front group A is immovable in FIG. 5, its image point is determined irrespective of the state of the focal adjustment, if the object point concerning the front group is fixed at one point. Since the image point of the front group can be deemed as the object point of the focusing lens group F, the aforementioned S.sub.F is determined only by the position of the focusing lens group F. Hence, the infinitesimal change .DELTA.S.sub.F of S.sub.F becomes equal to the infinitesimal movement .DELTA.d of the focusing group F, as is expressed by .DELTA.d=.DELTA.S.sub.F. Into this equation, the equation .circle.1 is differentiated and substituted: ##EQU2## Into this equation, .DELTA..beta..sub.F is substituted from the equation .circle.3 , following by the rearrangement: ##EQU3##
In respect to the rear group B having a positive power, on the other hand, the distance between the focal point and the object point is indicated as m, and the distance between the focal point and the image point is indicated as m'. Then, the well-known formula m.multidot.m'=-f.sub.B.sup.2 (wherein f.sub.B indicates the focal length of the rear group) is differentiated to .DELTA.m=f.sub.B.sup.2 /m'.sup.2 .multidot..DELTA.m'. into which the formula m'/f.sub.B =.beta..sub.B (wherein .beta..sub.B indicates the magnification of the rear group) also well known in the art is substituted: ##EQU4## Here, the infinitesimal change of the object point of the rear group B is equal to that of the image point of the focusing lens group F, and the infinitesimal change of said image point becomes equal to that of D.sub.F because the image point of the front group A, i.e., the object point of the focusing lens group F is determined. That is to say, .DELTA.m=.DELTA.D.sub.F. Since the rear group B is immovable, moreover, its image has an infinitesimal change .DELTA.L equal to that of m'. Hence, .DELTA.m'=.DELTA.L.
Hence, ##EQU5## From the equations .circle.3 ' and .circle.4 : ##EQU6##
FIG. 6 shows the state (FIG. 6(a)), in which the internal focusing type interchangeable lens is focused at an infinite distance, and the state (FIG. 6(b)) in which the focusing lens group F is moved therefrom by a distance X. If the state of FIG. 6(b) is applied to the equation .circle.5 showing the general state, then: ##EQU7## wherein: K.sub.o (X) indicates the ratio of the infinitesimal movement of the focusing lens group to the infinitesimal displacement of the focal point; and
If the state of FIG. 6(a) and FIG. 6(b) is applied to the aforementioned equation .circle.2 , moreover, then: ##EQU8## If the .beta..sub.FX from the equation .circle.6 is substituted into the equation .circle.5 , then: ##EQU9##
Here, the following relation holds among the focal length f.sub.A of the front group A, the magnification .beta..sub.F of the focusing lens group and their combined focal length f.sub.AF : EQU .function..sub.AF=.function..sub.A .multidot..beta..sub.F .circle.8
Likewise, the following relation holds among the combined focal length f.sub.AF of the front group and the focusing group, the combined focal length f of all the groups and the magnification .beta..sub.B of the rear group: EQU .function.=.function..sub.AF .multidot..beta..sub.B .circle.9
The equation of f.sub.AF =f.sub.A .beta..sub.F, which is made by applying the equation 8 into FIG. 6(a), and the equation .circle.9 are substituted into the equation .circle.7 and are rearranged. Then, the following relative equation of the internal focusing type lens is obtained: ##EQU10## As can be understood from the equation .circle.10 , the relation between the infinitesimal movement .DELTA.d of the focusing lens group F and the infinitesimal displacement .DELTA.L of the image becomes a function of the movement X of the focusing lens group F from the infinite focal point. In other words, the displacements of the image are different even if the focusing lens group is moved an equal distance, in the cases of the focusing states of long distance and short distance.
If the K.sub.o (X) is also determined for the rear focusing type interchangeable lens, the following equation is obtained by setting .beta..sub.B =1 in the equation .circle.5 because it is possible to assume that the magnification of the rear group of the internal focusing type is 1: ##EQU11##
FIG. 7 shows the state (FIG. 7(a)), in which the rear focusing type interchangeable lens is focused at infinity, and the state (FIG. 7(b)) in which the focusing lens group F is moved therefrom by the distance X. The state of FIG. 7(b) is applied to the equation 11, then: ##EQU12## If the state of FIG. 7(a) is applied to the foregoing equation .circle.2 , then like the internal focusing case the following equation holds: EQU X=.function..sub.F (.beta..sub.F .infin.-.beta..sub.FX) .circle.6
Hence, from the equations .circle.6 and .circle.11 ', the following equation is obtained: ##EQU13## Like the internal focusing case, too, the equation of f.sub.AF =f.sub.A .multidot..beta..sub.F obtained from the equation .circle.8 is substituted into the equation .circle.12 . Then, the following relation in the rear focusing type lens is obtained: ##EQU14## As can be understood from the equation .circle.13 , the relation of .DELTA.d and DL is a function of X, too, in the rear focusing interchangeable lens.
Thus, the ratio of the infinitesimal displacement of the image to the infinitesimal let-off of the lens group is substantially fixed in the total let-off type interchangeable lens but is the function of X, as is expressed by the equations .circle.10 and .circle.13 in the internal or rear focusing type interchangeable lens.
Those equations .circle.10 and .circle.13 are plotted in the graph of FIG. 8. These graphical representations are made by substituting the actual numerical values into the equations for the respective total let-off, internal focusing and rear focusing type interchangeable lenses. Therefore, the numerical values become variously different depending upon the lens structures, but the trends of the graphs have similarities for the focal adjusting types. For the internal and rear focusing types, as is understood from the graphs of FIG. 8, the movements required of the focusing lenses are different even for the equal displacement of the focusing position in the long shot and close-up photographies and have to be corrected. As a result, the coefficient corresponding to the photographic distance has to be transmitted as a variable amount of information from the interchangeable lens to the auto-focusing device, and the interchangeable lens mount has to be equipped with an encoder and a variable resistor so that the lens mount is large-sized, complicated and has a raised production cost. In addition, the information such as the aforementioned coefficient has to be transferred between the camera body and the interchangeable lens so that contacts or the like are required to further increase the complexity. There arises another problem that the precision is degraded by the limit to the resolution of the encoder itself or by the mounting error of the variable resistor.